Cryogenic Ion Mobility-Mass Spectrometry Captures Hydrated Ions Produced During Electrospray Ionization

Evaporation of water from extensively hydrated protons and peptides formed by electrospray ionization (ESI) has been examined for the first time by cryogenic ion mobility-mass spectrometry (IM-MS). The extent of hydration was controlled using a heated capillary inlet operated between 340 and 391 K. Cold cluster ions formed in the source region were transported into a low temperature (∼80 K) IM drift tube using an electrostatic ion guide where they were separated on the basis of size-to-charge via low-energy collisions with helium gas. The eluting IM profile was subsequently pulsed into an orthogonal time-of-flight (TOF) mass spectrometer for mass-to-charge (<i>m</i>/<i>z</i>) identification of the cluster ion species. Key parameters that influence the cluster distributions were critically examined including the inlet temperature, drift tube temperature, and IM field strength. In agreement with previous studies, our findings indicate that water evaporation is largely dependent upon the particular charge-carrying species within the cluster. IM-MS results for protonated water clusters suggest that the special stability of H⁺(H₂O)_<i>n</i> (<i>n</i> = 21) is attributed to the presence of a compact isomer (assigned to a clathrate cage) that falls below the trendline produced by adjacent clusters in the <i>n</i> = 15 to 35 size range. Peptide studies are also presented in which specific and nonspecific solvation is observed for gramicidin S [GS + 2H]²⁺(H₂O)_<i>n</i> (<i>n</i> = 0 to ∼26) and bradykinin [BK + 2H]²⁺(H₂O)_<i>n</i> (<i>n</i> = 0 to ∼73), respectively

From Solution to the Gas Phase: Factors That Influence Kinetic Trapping of Substance P in the Gas Phase

Substance P (RPKPQQFFGLM-NH₂) [M + 3H]³⁺ ions have been shown to exist as two conformers: one that is kinetically trapped and one that is thermodynamically more stable and therefore energetically preferred. Molecular dynamics (MD) simulations suggested that the kinetically trapped population is stabilized by interactions between the charge sites and the polar side chains of glutamine (Q) located at positions 5 and 6 and phenylalanine (F) located at positions 7 and 8. Here, the individual contributions of these specific intramolecular interactions are systematically probed through site-directed alanine mutations of the native amino acid sequence. Ion mobility spectrometry data for the mutant peptide ions confirm that interactions between the charge sites and glutamine/phenylalanine (Q/F) side chains afford stabilization of the kinetically trapped ion population. In addition, experimental data for proline-to-alanine mutations at positions 2 and 4 clearly show that interactions involving the charge sites and the Q/F side chains are altered by the cis/trans orientations of the proline residues and that mutation of glycine to proline at position 9 supports results from MD simulations suggesting that the C-terminus also provides stabilization of the kinetically trapped conformation